1. Field of the Invention
The invention relates to a protection system for tension members of a structure, in particular for a stay cable of a bridge structure, for collecting ice accumulations detaching from a tension member.
2. Description of the Background Art
In building construction, tension members are used primarily to suspend components, for example stay cables for suspending a bridge girder of a bridge structure on a pylon or for staying overhanging components, for example a roof on a foundation. For this purpose, the tension members are anchored by their ends in abutments on the components. The area between the abutments is exposed to the outdoors and is therefore subjected to the influences of the weather.
Under special climatic conditions, such as high relative humidity combined with frequent changes in temperature or freezing of damp snow, there is the danger of ice formation and ice accumulations over the circumference and along the length of the tension members. Upon reaching a critical size and weight, or due to external influences such as wind, thaws, shocks and the like, the ice accumulations run the risk of becoming detached and falling to the ground from a great height and causing injury/damage to people and property. This problem arises in particular in cable-stayed and suspension bridges, in which the traffic flowing beneath the cables is directly exposed to falling ice accumulations, and often only closing the bridge structure remains as a last resort to avoid damage.
To remove ice accumulations on the outer circumference of tension members, it is known for workers to carry out this work manually on site, which, however, is a very exhausting and time-consuming task. In addition, wind and weather must permit such deployment, which means that a dependency on the weather conditions exists. As a result, there has been no lack of effort in the past to develop other means of deicing tension members that are exposed to the outdoors.
Mobile, autonomous deicing devices have been used, which encompass and are movable along a tension member. With the aid of removal tools such as brushes and scrapers on the device, adhering ice accumulations are removed from the tension member.
In another deicing attempt, pressure waves were applied to the stay cables of a bridge structure by means of the rotor of a helicopter. Part of the ice accumulations detached from the stay cables and fell to the ground.
A device and a method are known from U.S. Pat. No. 6,518,497 B1, in which a tension member is subjected to a defined twisting motion over its length. The ice adhering to the surface of the tension members is unable to follow the resulting deformations and therefore breaks off.
JP H05-33 311 A discloses a hollow cylindrical carriage for removing snow accumulations, which movably rests on the cable and has drive wheels and cutting tools. The drive wheels set the carriage in a helical motion running along the cable, cutting tools which are in engagement with the snow accumulations freeing the cable of snow accumulations.
An arrangement for removing ice from high voltage overhead lines is described in DE 1 216 956 A, in which each conductor cable is surrounded by an elastic insulating sleeve. A hose is inserted between the conductor cable and the insulating sleeve, to which compressed air may be applied as needed. The associated increase in volume generates a blasting effect on the ice accumulations, so that they break away.
What the aforementioned approaches have in common is that ice accumulations become detached during the course of deicing and fall down, so that the area beneath the tension members must be protected, at least for the duration of the deicing measure.
In another approach, thermal systems are used, in which resistance heating wires are integrated into the tension member. The application of electrical energy to the resistance heating wires is intended to cause ice accumulations to melt. The disadvantage in this case is the enormous energy demand to be expended.
In addition, passive protective systems exist in the form of hydrophobic coatings, which are intended to prevent the adhesion of ice accumulations, and/or thermally absorbent coatings, which heat up due to heat absorption and cause the ice accumulations to melt. Although passive systems are economically superior to the systems mentioned at the outset, they are unable to reliably eliminate the danger of ice forming on tension members.